Ternary B-Site Alloying and Structure–Property Relationships of Two-Dimensional (iso-BA) ₂ Pb _<i>x</i> Sn _<i>y</i> Ge _{1– <i>x</i> – <i>y</i>} I ₄ Perovskites

The dependence on toxic lead (Pb), together with the trade-off between reduced toxicity and optoelectronic performance in single-metal B-site-substituted perovskites, remains a central obstacle to perovskite commercialization. Although two-dimensional (2D) perovskites exhibit greater compositional tolerance than three-dimensional (3D) counterparts, the full ternary Pb–Sn–Ge B-site compositional space remains largely unexplored, and a systematic structure–property phase diagram remains unestablished. Herein, a compositional-gradient series of (iso-BA) 2 Pb x Sn y Ge 1– x – y I 4 2D perovskite solid solutions is synthesized. Combining multimodal characterization with first-principles calculations, we establish the first systematic compositional phase diagram of this system, clarify the intrinsic limits of solid-solution formation, and reveal the origin of the composition-dependent bandgap evolution. Continuous bandgap tunability over 1.33–2.19 eV is achieved, and the low-Ge single-phase region is identified as the optimal window balancing low toxicity and favorable optoelectronic performance. This work provides a general framework for exploring multication low-dimensional perovskites and enables the rational design of low-toxicity, high-performance optoelectronic materials.